1. Technical Field
The present invention relates to thermal printers, and more particularly to precisely measuring the movement of media along a media transport path.
2. Background Art
Color thermal printers form a color print by successively printing with a dye donor onto a dye receiver, where the dye donor includes a repeating series of color patches. The print head of a thermal printer commonly provides a print line of elements that can be individually heated. Print heads can be any one of several forms including resistive element, resistive ribbon and laser print heads.
FIG. 1 shows a typical printing operation where a printer 10 includes a print head 12 and a platen 14. A dye donor 16 and a dye receiver 18 are sandwiched between the print head and the platen. An image is printed by selectively heating individual elements of print head 12 to transfer a first dye to dye receiver 18. The dye receiver is then repositioned to receive a second color of the image, and the dye donor is positioned to provide a second dye color. These steps are repeated until all colors of the image are printed and the completed print is ejected from printer 10.
The alignment of each dye donor color patch to the print head is important to achieve a quality print. Alignment refers to locating two independent components in specific positions with respect to each other. There are at least two approaches for aligning the dye donor color patches to the print head. One such approach is shown in U.S. Reissue Pat. No. RE 33,260, and uses color sensors to detect the color of a color patch and to emit a distinctive color-type signal when an edge of a color patch passes the color sensors. The accuracy of positioning a color patch to the print head is directly related to the location of the color sensors with respect to the print line of the print head. Putting the color sensors at the print line requires locating the color sensors off to one side of the print head, which in turn requires wider dye donor material, as depicted in FIG. 2. This method uses dye donor 16 inefficiently because of the additional width which cannot be used for making a print, resulting in increased cost per print for the user.
Locating the color sensors upstream or downstream of the print line avoids the need for wider dye donor. In FIG. 3, color sensors 20 are located downstream of print head 12. Thus, when the leading edge of a color patch is sensed, the print line is located within the color patch. If dye donor 16 is not moved after the leading edge of the color patch is sensed, the amount of dye patch between the print line and the color sensors is unused. This presents a problem due to the distance between color sensors 20 and the print line of print head 12. Dye donor 16 is again wasted unless it is rewound prior to printing. This undesirable waste of dye donor 16 again increases the cost per print for the user.
The dye donor could be rewound after the leading edge is sensed to reduce the unused portion of each color patch. This method has two disadvantages. First, an additional motor and media transport component would be needed to drive the donor in the reverse direction, significantly increasing the cost and complexity of the printer. Second, because the accuracy with which the dye donor can be rewound is uncertain, the dye donor must be rewound an amount less than the separation of the color sensors to the print line to insure that the print line remains within the color patch. This requires accurate metering of the donor movement. Metering in this case is the measurement of distance between two locations. Accurate rewinding of dye donor 16 requires a complex bidirectional donor transport system and an accurate metering method to measure how far dye donor 16 has been moved. This metering can be provided by adding an encoder or timing wheel to either the donor supply spool or the donor take up spool. One example of this method is shown in FIG. 4, where an encoder 26 is attached to a dye donor supply spool 22. As supply spool 22 rotates, an encoder sensor 28 responds to the motion of encoder 26 and outputs appropriate signals to determine how far the donor 16 has moved.
These methods suffer from two disadvantages. First, the amount of dye donor 16 movement for one rotation of spool 22 depends upon the donor diameter. In other words, more media moves for one revolution of a new donor supply spool 22 than for a nearly spent supply spool 22. It is difficult to know the diameter of donor on spool 22 without yet more sophisticated and expensive components. Thus, accurate measurement of dye donor 16 movement is not provided. An additional disadvantage of these two methods is that both add significantly to the cost and complexity of the printer hardware.
The color sensors could also be positioned upstream of the print line. This solution eliminates the need for rewinding the donor after the edge of the color patch is sensed. However, it requires accurate metering of the donor some amount greater than the separation of the color sensors from the print line, to insure the print line is within the color patch for printing. Hence, this method also has the disadvantage of requiring additional expensive components for its implementation.
Whether the color sensors are located upstream or downstream of the print line, the color patch size must be larger than the maximum size image to allow for color patch alignment tolerances. The patch size increase is related to the accuracy (or inaccuracy) of donor movement and can be a significant percentage of the actual printed image size. This results in inefficient usage of donor, caused by an inability to move media a precise distance, and resulting in an increased cost per print.
The second major approach for aligning a color patch to a print head utilizes a detectable mark provided on the dye donor to indicate the start of a color group or color patch. A detection mark is a symbol or collection of a small number of marks, such as a bar code, which conveys information. Detection marks may be produced using optical, magnetic, electrical, tactile or any other method that is easily readable. One example of this method is shown by Maeyama et al. in U.S. Pat. No. 4,496,955.
Maeyama et al. show a dye donor with two series of detection marks. The first series of detection marks identifies the beginning of a color group and the second series identifies the beginning of each color patch. Two detection mark sensors, one for each series of marks, are located downstream of the print line. In the operation of Maeyama et al., the donor is fast forwarded at the completion of printing a color patch. When a detection mark is sensed, positive drive tension is removed from the donor, after which the donor continues to coast in a forward direction. Some time later, when a mechanical sensor is activated by the platen movement, the signals from an encoder attached to the platen are counted until the platen has moved to the first printing position. The detection marks in Maeyama et al. provide dye donor velocity control signals, and are not directly used to align color patches to the print line or to measure the amount of donor movement. The accuracy of this method may be affected by lifting of the print head when the dye donor advances between color patches. If the print head in Maeyama et al. remains pressed against the platen during the printing of all color patches, it may be assumed that the motion of the platen is closely related to the motion of the donor. Dye donor often is distorted by the heating it receives during printing, thus this donor-platen motion relationship may not always be equal. Other thermal printers release the pressure of the print head against the platen between printing with individual color patches. When this is done, the relationship between platen movement and dye donor movement is lost. Hence accurate dye donor movement would not be provided with the Maeyama et al. implementation.
Ito et al. U.S. Pat. No. 4,720,480 describes numerous ways to provide a detection mark on dye donor and dye receiver. The examples presented by Ito et al. are directed to a single detection mark for each color patch or region, located near the beginning of a color patch or color group. This detection mark provides information confirming the region of a desired color in a color dye donor, confirming residual number of sheets in a monochromatic dye donor, or otherwise confirming the front or back, direction, grade, etc. of the dye donor. No indication is given that any of these detection mark forms are used for accurately measuring the movement of the dye donor. Ito et al. also describe providing a detection mark on dye receiver to supply the same types of information as the dye donor. Again, these detection mark forms are not used for accurately measuring the movement of the dye receiver.
The measurement of dye donor or dye receiver position rather than their movement is inherent in the detection mark concepts decribed thus far. Other efforts have been made to provide precise movement of dye donor or dye receiver, sometimes known as metering.
Shimizu et al. describe in U.S. Pat. No. 5,037,218 a method that combines a detection mark on dye donor with several sensors and encoders to provide accurate metering of dye donor. The detection mark sensed by a first sensor identifies the dye donor type and its sequence of color patches. A signal generator mechanically linked to the platen produces a first set of signals related to the print line spacing of an image. A second sensor generates a second set of signals related to the turning of an encoder attached to the dye donor supply spool. After the detection mark is sensed, the printer compares the first and second sets of signals to determine how much of the dye donor remains on the supply spool. When the first color patch has been printed and more than half of the dye donor is on the supply spool, the dye donor is moved to the next color patch by driving the supply spool for one revolution. However, when less than half of the dye donor is on the supply spool, the dye donor is moved to the next color patch by driving the supply spool for two revolutions. The Shimizu et al. metering method approximately positions the dye donor for all color patches, and does not provide accurate measurement of dye donor movement or positioning of the print line in the color patch. Larger color patch sizes are still required to allow for variation of the printed image area within a given color patch. As with the other encoder methods discussed before, Shimizu et al. require many more components in a significantly more complex hardware implementation than is necessary or desirable. All of these difficulties increase the complexity and cost of the printer and the per print cost to the user, without providing accurate metering or dye donor alignment.
Finally, Takanashi et al. describe a dye receiver metering method in U.S. Pat. No. 4,590,490. The dye donor, dye receiver and print head in Takanashi et al. are significantly larger than the final printed image. When the first color patch information in printed onto the dye receiver, synchronization marks are printed along a border of the dye receiver, outside the printed image area. The Takanashi et al. implementation requires a print head which is significantly larger than the printed image, or, alternatively, not all of the print head is utilized to print the image. Synchronization mark sensors are located at the print line, further increasing the overall size of the dye donor and dye receiver necessary for this method to function. The print head design is much more complex than common designs and inefficiently uses the printing elements available on the print head. The synchronization mark sensors at the end of the print head have the same problems as decribed in FIG. 2 earlier. The Takanashi et al. method requires significantly larger dye donor and dye receiver, wasting a significant proportion of both and requiring the user to remove the unwanted synchronization marks after printing is complete. Takanashi et al. use synchronization marks to indicate where the initial color patch print lines were printed. These marks, applied by the print head during the printing operation, are not used to measure the movement of the dye receiver. All of the problems mentioned here significantly increase the complexity and cost of the printer, difficulty for the user to make a print, and increases the cost per print to the user. None of these are beneficial.
None of the preceeding prior art methods provide accurate movement or alignment of the dye donor or dye receiver in the printer. All require more complex hardware and less efficient utilization of the dye donor or dye receiver. These methods undesirably impact the cost of the printer and the cost per print to the user.